Veterinary Neuropharmacology

CATABOLISM OF NEUROTRANSMITTERS

• Norepinephrine is metabolized both by intraneuronal and extraneuronal enzymes.
• However the uptake of norepinephrine into the adrenergic nerve terminal and diffusion of norepinephrine away from the receptor sites are the important pathways of termination of activity.
• Extraneuronal acetylcholine is rapidly hydrolyzed by acetylcholinesterase, a quite specific enzyme localised in close proximity to the synaptic cleft.
• Nitric oxide is a highly reactive free radical that undergoes oxidation within seconds to nitrites and nitrates.

Note

• Nerves releasing acetylcholine are termed as cholinergic and nerves releasing nor epinephrine are termed as adrenergic.
• In addition to these nonadrenergic non cholinergic (NANC) transmission also occurs.
• The transmitter responsible for transmission at NANC is nitric oxide. NANC has been demonstrated in gut, vas deferens, urinary tract, salivary glands and certain blood vessels.
• The supply to genitalia is parasympathetic, but acetylcholine is not the transmitter. It may be nitric oxide that is responsible for penile erection.